The present invention relates generally to a distance measuring device and a method for determining a distance.
For the detection and recording of the position of the piston of fluidic linear drives or pneumatic and hydraulic cylinders as the conducting structure, it is generally known to use distance measuring devices measuring the distance between the piston serving as a reflecting body inside the conducting structure and the cylinder lid or bottom of the conducting structure. The detection of the piston position in cylinders can be performed either discrete, i.e. at discrete sites, or continuous, i.e. continuously during operation.
A discrete determination of piston position is usually applied in order to feedback the performance or completion of a piston movement to a process control (e.g. SPS), for example in order to be able to initiate the next process step. Magnetic field-sensitive sensors or sensor facilities are used predominantly for this purpose, which detect the magnetic field of a permanent magnet residing on the cylinder piston. The sensors used for detection are mounted to the outside of the cylinder tube. When the piston moves into the detection area of a sensor of this type, the sensor detects the presence of the cylinder piston through the cylinder tube. In the majority of cases, this requires the use of non-ferromagnetic materials and thus limits the design properties and/or applications of the drive. However, if a different position of the piston is to be detected, the sensor needs to be mechanically adjusted or newly adjusted accordingly. Therefore, an additional sensor must be mounted for each additional position to be detected, which is associated with additional material, mounting, adjustment, and installation costs in each case.
Another disadvantage is that the mounting usually is performed on customer premises. The situation is aggravated if the cylinder is already assembled in a machine that is difficult to access such that it may be impossible to adjust the sensing distances by mechanical shifting of the externally mounted magnetic switches. Moreover, these externally mounted sensors require additional space. Often additional design work is required in order to provide for the accessibility and robustness of the sensor.
Sensors of this type are implemented predominantly in the form of magnetic field-sensitive sensors and are known as Reed switches, magneto-resistive (MR), giant magneto-resistive (GMR), Hall switches or magneto-inductive proximity switches. However, the detection of the magnetic field requires extensive adjustment of the magnet to the sensor and/or sensor facility. Moreover, this measuring principle limits the possible applications because of interfering static or dynamic magnetic fields (EMV, field of a cylinder in close proximity) and the temperature properties of the sensor.
For continuous piston position measurement, it is common to use measuring systems based on potentiometry, the linear variable differential transformer (LVDT) principle or the ultrasound principle. In these systems, the position of the piston is output continuously and, in the majority of cases, as an analogous voltage signal. Sensors working according to the LVDT principle always require a calibration run for pre-adjustment after being turned on. Ultrasound sensors are suitable only to a limited degree for path length measurements in pneumatic or hydraulic cylinders, since the accuracy of measurement changes with cylinder pressure. Incremental path length measurements supplementing these systems are also known. These systems are implemented for example by coding the piston rod and thus can be used only for relative path length measurements.
In summary, both the continuous and the discrete determination of piston position according to the prior art can be integrated into a cylinder either not at all or only with substantial design efforts which are associated with high costs. Substantial design efforts are required because all common sensor principles described above must be adjusted to the respective cylinder length, since their detection area is too short.
It is desirable to create a distance measuring device and a method for determining the distance, which overcome the disadvantages listed above and provide for continuous determination of the distance which can therefore also be made discrete and in addition is easy to handle and provides for a large variety of applications.